Semiconductor technology and the associated packaging and diagnostic techniques have advanced dramatically over the past decade. Many of the most advanced and complex logic IC's such as microprocessors, digital signal processors and so-called SOC's (System-On-a-Chip) are today commonly packaged facedown using flipchip mounting technology on a substrate rather than conventional face-up wire bonded packaging. Flipchip packaging has many advantages including accommodation for very high I/O pin counts, low inductance signal-paths for high speed signals, low resistance power distribution, small form-factor and potentially low-cost packages.
However, flipchip packaging presents substantial challenges for design debug, prototype modification and failure analysis as the active circuitry is directly mated with the package substrate and cannot readily be accessed for probing or modification without compromising the package signal integrity (thus severely hindering any functional electrical diagnostic work and particularly at-speed analysis).
Diagnostic electrical probing and prototype repair or modification remain critical parts of the commercial race to get new IC products to market quickly. Techniques exist for through-the-substrate probing and modification of flipchip packaged IC's. These include pulsed laser IR timing waveform probing through a thinned silicon substrate using products such as the IDS 2000 and the IDS 2500 from NPTest, Inc, of San Jose, Calif. Other techniques include use of FIB (Focused Ion Beam) with halogen chemistry acceleration for cutting trenches and holes in IC substrates using products such as the IDS P3X also from NPTest of San Jose, Calif.
Improvements in cost, complexity and ease of operation of these techniques are highly desirable. As the number of metal interconnect layers has increased and with the prevalence of using the top metal layers as power-planes, even some conventionally wired bonded IC's are now being probed, diagnosed and modified using techniques adapted for use through the backside of the IC substrate.
Processes for backside Focused Ion Beam (FIB) operations have been developed and are in use at a few advanced IC manufacturers today. Examples of such processes are described in U.S. Pat. No. 5,821,549 (hereafter '549) to Talbot et al. “THROUGH THE SUBSTRATE INVESTIGATION OF FLIP-CHIP IC'S”, issued Oct. 13, 1998, and in U.S. Pat. No. 6,069,366 (hereafter '366) to Goruganthu et al. “ENDPOPINT DETECTION FOR THINNING OF SILICON OF A FLIPCHIP BONDED INTEGRATED CIRCUIT”, issued May 20, 2000, both of which are incorporated herein by reference. These processes typically use a flow similar to the one that follows:                preparation of a substrate by thinning with mechanical polishing or sometimes chemical mechanical polishing from approximately 800 um thickness down to approximately 20-200 μm.        coarse cutting of a trench with Laser Chemical Etching (LCE) to within approximately 5-20 μm of the active front surface of the silicon        coarse, chemically assisted FIB milling of a smaller trench within the LCE trench to within one to a few microns of the active diffusion regions (the chemical assistance is typically achieved using halogen-based chemistry injected into the vacuum chamber close to the operation site)        FIB sputter removal or fine chemically assisted FIB milling between active diffusion regions or active devices including transistors, diodes, etc., to provide access to one or more circuit elements, and        finally probing, cutting, depositing, or connecting signal paths as required.        
Two portions of this process are particularly error-prone, specifically the ability to stop the coarse and fine milling steps before active diffusion regions are breached or destroyed, and the positioning of the fine FIB milling operation to ensure again that active diffusion regions are not accidentally damaged. These errors in milling and positioning usually result in a non-functional or impaired device. Depending on the complexity of the process and the skill of the operator, success rates for this type of operation are relatively low and range from 50% to 90%. As the complexity and number of steps in a given modification sequence increases, the overall yield decreases as the product of the yields of each of the individual steps and quickly tends to zero for many of the more complex sequences.
There are a number of techniques for FIB operation endpoint detection in use, including monitoring sample stage current, monitoring a secondary electron detector signal, monitoring a secondary ion detector signal, monitoring a secondary ion mass spectrometer signal and even monitoring a photo-emission signal from excited secondary particles. These techniques all rely on a signal change at a material boundary or interface and are difficult to apply to backside operations where milling often must be reliably stopped before diffusion regions are perturbed, where there is no meaningful materials interface but merely a change in doping impurity concentration.
Other endpoint detection techniques are described in U.S. Pat. No. 5,140,164 to Talbot et al. “IC MODIFICATION WITH FOCUSED ION BEAM SYSTEM” issued Aug. 18, 1992, and in U.S. Pat. No. 5,948,217 to Winer et al. “METHOD AND APPARATUS FOR ENDPOINTING WHILE MILLING AN INTEGRATED CIRCUIT” issued Sep. 7, 1999. The approaches taught in these patents have the disadvantage of requiring that a signal (AC in the first case and DC in the second case) be applied to the circuit element being accessed.
Patent '366 presents one approach for more reliably stopping milling prior to exposing active diffusions using Optical Beam Induced Current (OBIC). A bright laser light source is used to sense the remaining material thickness. Laser light photons create electron-hole pairs in the substrate. As the milling process approaches the active diffusion regions, an increasing number of electron-hole pairs are generated in or near the active diffusion region of transistors and results in an increased leakage or photo-current that is monitored through the power supply pins of the IC being operated on. Based on characterization and experience, the operator is then able to perform more accurate endpoint detection and reliably stop the milling process prior to active diffusion region damage by periodically monitoring the optically induced leakage current.
The disadvantage of this approach is the need for a powerful laser light source. Patent '366 references the use of a relatively high powered 4W green laser (with a photon energy greater than the silicon band gap in order to create electron-hole pairs in the bulk silicon). Not all of this power would be focused onto the operation area. Although in principle the laser could be made to illuminate the operation site simultaneously with the milling operation, in practice this is quite challenging and commercial FIB systems today are not so equipped. Another challenge is gallium staining of the milled surface resulting in variations of the amount of light transmitted. Yet another challenge is the laser light interfering with, swamping or possibly even damaging the sensitive charged particle detector in the FIB system. From a practical standpoint today, either the IC or light source is periodically moved to monitor changes in the OBIC signal. This approach is cumbersome and time consuming and does not readily lend itself to a practical or fast closed loop endpoint detection system.
Patent '366 also suggests the use of an ion Beam Induced Current (BIC) signal for endpoint detection. The ion BIC signal from a typical ion beam milling current of approximately 1-10 nA is very much smaller than the OBIC signal produced by a 4W laser, and extremely poor signal-to-noise ratio has so far precluded its use in all but experimental situations.